In healthcare, teams are frequently composed of interchangeable members, and health care providers may find themselves working with new or different team members on a daily or even hourly basis. Teams with such unstable membership have been termed fluid by organizational psychologists.1 In our systems approach to healthcare safety, we have primarily focused on the limitations induced by fluid teams including loss of team cohesion, inefficient or inadequate transfer of information, and lack of a shared mental model leading to decreased efficiency and compromised patient safety. However, team fluidity exists in organizations for a reason and can provide advantages to system functioning. Those most obviously relevant in the OR include efficiency in adapting to scheduling availability and the need to train future healthcare workers. Other, less obvious, advantages include the ability to avoid collusive behavior and the opportunity to maintain high levels of vigilance. Such advantages of fluid teams have not been previously explored in the operating room or other healthcare settings. While it currently presents challenges to team efficiency and performance, team fluidity and the frequent addition of new team members represent an unrecognized opportunity for increasing vigilance and detecting unsafe situations. The applicant, Lane Frasier is a surgery resident who will have 2 years of protected research time to complete this F32 post-doctoral fellowship. This will allow Dr. Frasier to gain experience in research including patient safety, systems engineering, and health services research. The fellowship will include a comprehensive educational experience including didactic coursework, participation in a multi-disciplinary research program and mentored research project as described above. The mentors for this proposal are Dr. Caprice Greenberg, a surgeon and expert in patient safety and surgical performance, and Dr. Douglas Wiegmann, a cognitive psychologist and human factors engineer with expertise in the surgical domain. The University of Wisconsin, with its strong history of collaboration between surgery and engineering, offers the ideal environment for this innovative work. This project will not only provide the education and training necessary for Dr. Frasier to become a funded surgeon-scientist, but also generate critical new knowledge about team functioning in the operating room. These results will be used to develop training modules, best practices, and other interventions to optimize team fluidity and thereby improve team functioning, in the operating room, and other complex team- based settings in healthcare.